I usually use an app so I follow their instructions. But when I’m not using an app, it’s nice to shake it up any time I start to feel the urge to quit.
I normally change based on the time I have.If I have no time, I will look for a 5 min or 10 min exercise. If I have time I will go to a gym or do something outside. I try to never compare which it's better, I always assume that if I'm doing something that's enough. Another advantage it's that, since I vary the exercise depending on the time, I usually don't get bored of them, since it's naturally changing
Every few weeks or months. If I'm feeling burnt out, I'll give myself a few days and change it up to something lower stress or generally more managable.